PRE- AND SYN-KINEMATIC P-T CONSTRAINTS ACROSS A ZONE OF TRANSPRESSION, VERTICAL EXTRUSION, AND LATERAL ESCAPE IN THE SOUTHERN NEW ENGLAND APPALACHIANS

A model of late Paleozoic transpression, extrusion, and lateral escape has been proposed for the Bronson Hill (BHZ) and Central Maine (CMZ) Zones of southern New England, founded on bedrock mapping, structural and kinematic analyses, zircon and monazite geochronology, and the results of other workers. Thus far, constraints on the thermal architecture of the transpression system have been based on qualitative interpretation; however, this study investigates the P-T conditions before and during deformation with new petrologic modeling. Pseudosections in the MnNCKFMASH(T) system were constructed for five localities across strike of the of the transpression system in southern Massachusetts.

Structures and kinematics in the CMZ are representative of dextral transpression. The CMZ supracrustal rocks have been intruded by a suite of diorites and granites as young as ~355 Ma, and all lithologies were later subjected to partial melting. Pelitic paragneiss in the eastern interior of the CMZ contains leucosomes (Kfs-Pl-Qtz) aligned parallel to the foliation plane, and abundant lineation-defining Sil(II) that formed by the retrograde reaction Kfs+Grt+H2O -> Sil+Bt during dextral deformation. Sil(I) is included in Grt and there is no Ms in this sample, which could suggest initial Ms+Qtz+Pl -> Sil+melt followed by Bt dehydration to produce Grt including Sil(I). Further west in the CMZ, the pre- and syn-kinematic assemblages are similar. The presence of Crd suggests an additional reaction of Crd+Kfs -> Sil+Bt to produce Sil(II). In the same vicinity, a reverse top-to-east high strain zone is characterized by a synkinematic assemblage of Crd+Sil+Kfs+Gr with no retrogression.

The sinistral Mt. Dumplin high strain zone marks the western edge of the system, and accommodated lateral escape. Pelitic schist shows a peak assemblage of St-Grt-Ms-Qtz that is represented by porphyroclasts in various stages of synkinematic breakdown to Cld-Grt-Ms-Qtz-Chl and then Ms-Qtz-Chl.

At present, our works shows that dextral transpression in the CMZ was active at high T (>700 °C) and relatively moderate P (4-5 Kbar) after ~355 Ma. Lateral escape occurred at lower T (550 °C) and similar P. At present, we do not see resolvable differences in P, suggesting that vertical extrusion may not have been significant.